THEMIS observation of a substorm event on 04:35, 22 February 2008

Abstract. We report on THEMIS in-situ and ground-based observations during a substorm between 04:30~04:50 UT on 22 February 2008. The spacecraft (probes) were aligned along the tail between XGSM=−5 RE to −25 RE. The most distant probe P1 (X=−24.5 RE) detected two successive tailward moving bipolar magnetic structures. P2 (X=−18 RE), P3 (X=−11 RE), P4 (X=−10.5 RE) all captured signatures related to the Earthward movement of a magnetic structure. THEMIS ground stations and all-sky imagers also recorded Pi2 pulsations and a sudden brightening in a white-light auroral imager followed by poleward expansion. We perform a detailed timing analysis of probe and ground-based data and reconstruct the time sequence of phenomena during this substorm. The earliest sign of substorm onset was the bipolar perturbation in the northward component of the magnetic field (interpreted as the result of reconnection onset) at P1 at 04:35:16 UT and corresponding magnetic perturbation at P2 at 04:35:14 UT. Auroral onset was seen at or before 04:36:55 UT, consistent with the visual onset of high-latitude magnetic pulsations at around that time. Earthward flows at P3 and P4 seen at ~04:36:03 UT, and dipolarization onset at ~04:36:50 UT, were observed at almost the same time as the ground onset signature, implying that near-Earth dipolarization happened in the aftermath of tail reconnection but not significantly ahead of the auroral intensification. Reconnection in the tail preceded ground onset and near-Earth dipolarization (current disruption) by ~2 min. Two reconnection pulses (the first one weaker than the second one) accompanied by correlative increases of cumulative magnetic flux transfer into the reconnection region were observed. A direct association of the reconnection pulses with two auroral intensifications can be made, suggesting that tail reconnection, like the auroral expansion, advances in steps rather than continuously.